Detection of UHE gamma rays from the Crab Nebula: Physical Implications

TL;DR

This paper analyzes ultra-high energy gamma-ray detection from the Crab Nebula to constrain the properties of particle acceleration and magnetic fields, providing new insights into the nebula's extreme energetic processes.

Contribution

It offers a parameter-free estimate of the magnetic field strength in the acceleration region based on gamma-ray observations, constraining models of particle acceleration in the Crab Nebula.

Findings

Magnetic field strength in the acceleration region is estimated to be ≤120 μG.

Detection of gamma rays up to 300 TeV constrains electron energy distribution.

Future observations above 300 TeV and 1 MeV will clarify acceleration conditions.

Abstract

The Crab Nebula is an extreme particle accelerator boosting the energy of electrons up to a few PeV ($10^{15} \ \rm eV$), close to the maximum energy allowed by theory. The physical conditions in the acceleration site and the nature of the acceleration process itself remain highly uncertain. The key information about the highest energy accelerated particles is contained in the synchrotron and inverse Compton (IC) channels of radiation at energies above 1~MeV and 100~TeV, respectively. The recent report of detection of ultra-high energy gamma-ray signal from the Crab Nebula up to 300~TeV allows one to determine the energy distribution of the highest energy electrons and to derive the magnetic field strength in the acceleration region, $B\leq120\rm\,\mu G$, in a parameter-free way. This estimate brings new constraints on the properties of non-thermal particle distributions and puts important constraints on the MHD models for the Crab Nebula, in particular on the feasible magnetization and anisotropy of the pulsar wind. The calculations of synchrotron and IC emission show that future observations with instruments allowing detection of the Crab Nebula above 300~TeV and above 1~MeV will clarify the conditions allowing acceleration of electrons beyond PeV energies in the Crab Nebula. In particular, one will (1) verify the hypothetical multi-component composition of the electron energy distribution, and (2) determine the magnetic field strength in the regions responsible for the acceleration of PeV electrons.